Laser direct writing of planar lightwave circuits

ABSTRACT

A laser beam with an annular or ring shaped intensity distribution, such as a TEM 01 * beam, is scanned across the surface of a photosensitive thin film to directly produce changes of refractive index in selected regions of the film. This method is suitable for producing channel waveguides for planar lightwave circuits, where the refractive index profile (solid curve) of such a waveguide is more uniform than the prior art refractive index profile (dashed curve) produced by a TEM 00  laser beam, and has a reduced bend loss.

FIELD OF THE INVENTION

This invention relates to the production of planar lightwave circuits(PLC's). More particularly, the invention relates to a method fordefining the light wave circuit or components of the circuit by directphotoinduced changes in refractive index of a thin film forming thebasis of the PLC.

BACKGROUND ART

There is an increasing demand for planar lightwave circuits for advancedoptical communications networks, optical sensing and also as the basisof other photonic devices, such as high frequency signal processors foruse in military and other applications.

The basis of most PLC's is a trilayer of optically transparent thinfilms deposited on a substrate of generally silicon or silica as shownin FIG. 1. The central, or core layer 1 of the sandwich structurenormally has higher refractive index n_(c0) than the refractive indexn_(c1) of outer cladding layers 2, 3, and this simple system is known asa planar waveguide. Light injected into the core layer undergoes totalinternal reflection at both core/cladding boundaries 4, 5 and isconfined in this transverse dimension, resulting in 1-dimensional lightguidance. However as a consequence of the constant refractive index inthe plane of the film total internal reflection is not possible, andlight spreads or diffracts laterally in the guiding layer. To impartuseful functionality to a planar waveguide, 2-dimensional light guidanceis required, and planar diffraction must be overcome by introducinglocal changes in the core layer refractive index. The light guides soformed are known as channel waveguides, the basic elements of PLC'S, andthe final product is a planar lightwave circuit which can exhibit a widerange of optical functions. An example of a simple device is aconcatenated Y-junction splitter where the signals from a single inputchannel are split evenly into a larger number of output channelsindependent of wavelength.

Currently, there are several classes of materials and processing methodswhich can be used to produce planar waveguides. These include silicaglass (using plasma enhanced chemical vapour deposition: PECVD, or flamehydrolysis: FHD); organically modified silicate glasses (ORMOSILs) andplastics produced via wet chemical synthesis and spin coating; and III-Vsemiconductors produced by MOCVD or MBE growth.

The waveguides in the PLC are defined by structuring or patterning ofthe refractive index in the plane of the film. In the microelectronicsindustry, the standard patterning technique is known as maskphotolithography. The first step in this process is to deposit anadditional thin film of photo resist onto the planar waveguide core,usually by spin coating. The photo resist film is then preferentiallyexposed to a broadband extended UV source through an amplitude mask suchthat a photochemical reaction is initiated below the high transmissionareas of the mask. The photochemical reaction changes the solubility ofthe photo resist enabling it to be removed by agitation in a suitablesolvent. Depending on whether negative or positive tone resist is used,the irradiated regions will remain or be removed respectively. The photoresist pattern may then be transferred to the waveguide core layer byremoving core material from the unwanted regions by a process such asreactive ion etching. Removal of the remaining resist and over claddingwith a low refractive index film completes the standard processing ofthe PLC. Two dimensional waveguiding is therefore achieved throughselective removal of the high index core material.

As an alternative to the photo resist technology, materials such asplastics, ormosils and some glasses can allow refractive indexpatterning to be achieved without the use of an additional photo resistlayer. In this class of materials, direct exposure generally to UVradiation initiates a photochemical reaction that raises the refractiveindex of the core material, enabling channel waveguides to be formed.The materials are generically described as photosensitive. The use ofmask photolithography on these films therefore results in the productionof PLC'S without the requirement for the deposition of an additionalphoto resist layer or any reactive ion etching or wet development.

The production of a suitable mask is however costly and time consumingand, particularly in the prototyping stage, limits the number of devicedesigns that can be tested. Recently it has been demonstrated that therequirement for a mask can be eliminated by using the laser directwriting (LDW) technique. This is described in M. Svalgaard, ‘Directwriting of planar waveguide power splitters and directional couplersusing a focused ultraviolet laser beam’, Electronics Letters, Vol. 33,No. 20, pp. 1694-1695, 1997. In contrast to standard maskphotolithography, in the LDW process the photosensitive planarwaveguiding film is accurately traversed under a focused laser beam tolocally expose the material and directly delineate the channelwaveguides without the use of a mask. The LDW process is very versatileand permits a wider range of structures than is generally possible byexposure through a mask. For example, using a mask the exposure will beuniform across the whole wafer, and hence the refractive index changeobtained cannot vary from one part of the waveguide structure to thenext. Using LDW, on the other hand, permits the exposure, and hence theinduced change of the refractive index, to be adjusted even over shortdistances permitting a wider range of waveguide structures to bewritten. Furthermore, the LDW process permits the waveguide pattern tobe changed from wafer to wafer because the generated pattern can beunder direct software control.

As with mask photolithography, depending on the waveguide material used,the laser induced photochemical reaction can either directly induce arefractive index change in the waveguiding material without furtherprocessing, the material system can be locally exposed and thensubjected to wet development, or a secondary layer of photo resist maybe used and the photo resist pattern transferred to the waveguiding corelayer. Therefore depending on the laser source, the process isapplicable to many different material systems e.g. glass, polymer,sol-gel, Ti:LiNbO₃. In addition, the use of high power lasers and/ordifferent laser wavelengths can access photosensitive mechanisms thatcannot be attained with mask photolithography.

In general the refractive index, n, of a photosensitive material isdependent on it's exposure, F, to electromagnetic radiation, and asshown in FIG. 2 the material response function, n(F), typically exhibitsa saturation behaviour up to a maximum exposure, F_(sat). A plan view ofthe laser writing process is shown in FIG. 3, where we considerdelineation of a channel waveguide of width 2 a in the z direction. Fora laser irradiance distribution, I(y,z), the lateral exposure of thematerial in the y direction, F(y), is given by; $\begin{matrix}{{F(y)} = {\frac{1}{v}{\int_{- \sqrt{a{({a - y})}}}^{\sqrt{a{({a - y})}}}{{I\left( {y,z} \right)}\quad {z}}}}} & (1)\end{matrix}$

where v is the writing velocity. The lateral exposure, F(y), for atruncated (beam waist, w=a) TEM₀₀ beam with irradiance distributionI(y,z)=I₀₀ exp(−2(y²+z²)/w²), is shown by the dashed line in FIG. 4.Clearly the exposure is peaked around y=0. For optimum use of theavailable refractive index change the maximum lateral exposure, F_(max)should equal the material saturation exposure, F_(sat). Taking intoaccount the material response n(F), the corresponding refractive indexdistribution n(y) also shows a maximum at y=0 as shown by the dashedline in FIG. 5. Laser direct write systems so far have employed thefocusing or imaging of weakly truncated Gaussian (TEM₀₀) beams, andoperating under this scenario therefore produce channel waveguides withlaterally graded refractive index profiles. Indeed, graded thicknessprofiles are often observed in wet developed polymer waveguides. This isdescribed in L. Eldada, L. W. Shacklette, R. A. Norwood and J. T.Yardley, ‘Next-generation polymer photonic devices’, Sol-Gel and PolymerPhotonic Devices, SPIE Vol. CR68, pp. 207-227, 1997. Graded indexprofiles have some disadvantages when factors such as bend loss and formbirefringence are taken into account.

DISCLOSURE OF THE INVENTION

It is an object of this invention to provide a method of directlyproducing photoinduced changes in refractive index that provides a moreuniform lateral refractive index distribution.

Accordingly, in one broad aspect this invention provides a method ofdirectly producing photoinduced changes in refractive index in selectedregions of a photosensitive thin film by selectively scanning a laserbeam across the surface of the film, characterised in that the laserbeam impinging the film has an annular substantially circularlysymmetric irradiation intensity distribution.

As used herein the term annular intensity distribution includes anytransverse intensity distribution that has a very low or zero intensityin a central region and a surrounding region of higher intensity. Theradial intensity distribution within the surrounding region can besubstantially uniform or may vary. This type of intensity distributionis sometimes referred to as a “doughnut type” distribution. Such adistribution is achieved, for example, by a TEM₀₁* laser beam althoughhigher order modes can also produce an intensity distribution havingsimilar characteristics. The TEM₀₁* is a hybrid mode consisting of asuperposition of TEM₀₁ and TEM₁₀ with a constant phase difference of±π2. The radial intensity distribution in a TEM₀₁* beam is given by$\begin{matrix}{{I\left( {y,z} \right)} = {{I_{01}\left( {\frac{y^{2}}{w_{1}^{2}} + \frac{z^{2}}{w_{2}^{2}}} \right)}{\exp \left( {{- 2}\left( {\frac{y^{2}}{w_{1}^{2}} + \frac{z^{2}}{w_{2}^{2}}} \right)} \right)}}} & (2)\end{matrix}$

where w₁ and w₂ are beam waists and w₁≈w₂

In the preferred form of the invention a TEM₀₁* laser beam is used. Thebeam, for example, can be generated from a Gaussian laser beam by adiffracting phase mask technique as described in N. R. Heckenberg, R.McDuff, C. P. Smith and A. G. White, ‘Generation of optical phasesingularities by computer-generated holograms’, Optics Letters, Vol. 17,No. 3, pp. 221-223, 1992. A TEM₀₁* beam can also be produced directlyfrom a laser by suitable modification to the cavity optics.

The use of a beam with an annular or doughnut type irradiancedistribution results in a lateral exposure F(y) that produces a moreuniform lateral refractive index distribution, n(y). This is achievedbecause I(y,z) is modified such that the contribution to the integral inequation (1) near y=0 is reduced. In addition, to avoid introducing apreferential writing direction the focused beam must remainsubstantially circularly symmetric.

The invention will be further explained, by way of example only, withreference to the accompanying.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a planar waveguide;

FIG. 2 illustrates the typical refraction index response of aphotosensitive medium as a function of exposure to electromagneticradiation;

FIG. 3 is a schematic planar view of a laser circuit writing processused in the method of this invention;

FIG. 4 illustrates the lateral exposure function of TEM₀₁* (solid line)and TEM₀₀ (dashed line) for equal maximum exposure writing scenarios;

FIG. 5 illustrates the lateral refraction index distribution for TEM₀₁*(solid line) and TEM₀₀ (dashed line) for equal maximum exposure writingscenarios; and

FIG. 6 illustrates bend loss for TEM₀₁* (solid line) and TEM₀₀ (dashedline) written waveguides of equal spot size and maximum exposure.

BEST MODE FOR CARRYING OUT THE INVENTION

The method of this invention utilises a laser direct write (LDW)technique in which a focussed TEM₀₁* laser beam is used. The LDWtechnique is known to those skilled in the art and will not be describedin detail.

The TEM₀₁* ‘doughnut mode’ has an irradiance distribution which exhibitszero intensity at the beam centre. The maximum exposure is constrainedto be equal to the material saturation exposure and results in thelateral refractive index distribution as shown in FIG. 5. The laserwriting with a TEM₀₁* beam gives a larger photoinduced refractive indexchange in the outer regions of the beam than that obtained with a TEM₀₀counterpart of equal F_(max). Furthermore, the change of index is moreuniform over the waveguide region. These features impart desirableproperties to the photoinduced waveguides. The more uniform lateralrefractive index profile generated by the ‘doughnut’ mode beam, hasseveral technical advantages when compared with LDW systems usingGaussian modes.

A practical consideration related to the general use of all PLC's isthat it is usual for the input and output ports to be connected to therest of the optical network with silica optical fibres. For maximumoptical power transfer from the fibre to the channel waveguidescomprising the PLC, the spot size and symmetry of the channel guidedmode must equal that of the circularly symmetric fibre guided mode. Fora given material system with a specified F_(sat) this excitationefficiency criterion determines the dimensions of the channelwaveguides. If the channel waveguides produced using either TEM₀₀ orTEM₀₁* mode LDW must have mode sizes that are matched to those of atypical communications fibre, and the same maximum exposure of thephotosensitive layer is used for both TEM₀₀ and TEM₀₁* LDW, then alarger diameter writing beam is required in the TEM₀₀ case. This isbecause, as shown in FIG. 5, the photoinduced refractive index changesare low in the outer regions of the beam for TEM₀₀ exposure. Thewaveguide must therefore be made physically wider to pull the opticalfield out to match the fibre mode field size.

In the weak guidance regime, the mode effective index can beapproximated as a weighted average of the refractive index distribution,n(x,y), seen by the mode, i.e. $\begin{matrix}{N_{eff}^{2} \approx \frac{\int{\int{{n^{2}\left( {x,y} \right)}{\psi^{2}\left( {x,y} \right)}\quad {x}\quad {y}}}}{\int{\int{{\psi^{2}\left( {x,y} \right)}\quad {x}\quad {y}}}}} & (3)\end{matrix}$

where ψ(x,y) is the mode function. Evaluating equation (3) for both theTEM₀₀ and TEM₀₁* cases gives a lower effective index value in the TEM₀₀case. Expressing this in terms of the normalized effective index, B,defined as; $\begin{matrix}{B = \frac{N_{ef}^{2} - n_{cl}^{2}}{n_{co}^{2} - n_{cl}^{2}}} & (4)\end{matrix}$

where n_(c1) is the cladding refractive index and n_(c0) is the maximumphotoinduced refractive index, gives a B value typically 20% higher inthe TEM₀₁* case. The channel guided light is, therefore, more stronglyconfined in TEM₀₁* written waveguides, under the constraints of maximumexcitation efficiency and F_(max)=F_(sat). The stronger confinementimparts significant technical advantages for the TEM₀₁* writtenwaveguides.

Useful functionality is only achieved in a PLC if the channel waveguidesare routed in curved paths from one region of the optical chip toanother, and this requires bending of the waveguides in the plane of thefilm. When a channel waveguide is bent, an amount of power, L, isinescapably lost to radiation per unit length of bend. L is stronglydependent upon the radius of curvature of the bend; the tighter thebend, the greater the bend loss. For a constant radius, the magnitude ofL is directly related to how strongly the guided mode is confined.Channel waveguides produced by TEM₀₁* LDW, therefore, exhibitsignificantly reduced bend loss compared to channel waveguides formed byTEM₀₀ LDW. This feature is illustrated in FIG. 6. The minimum bendradius (for 0.1 dB/cm bend loss) permissible for a TEM₀₁* written guidemay be as small as ˜75% the value of that possible using waveguideswritten by TEM₀₀ mode beams. This significantly impacts the devicedensity on the planar waveguide typically increasing by a factor of 1.8the number of devices that can be written on a given area of the optical“chip”.

In addition, the index profiles generated through TEM₀₁* LDW result inchannel waveguides with form birefringence reduced by typically a factorof 2 relative to TEM₀₀ written guides.

Variations in laser intensity or writing velocity (and thereforeexposure) during the laser direct writing process will result influctuations in the effective index of the optical guided mode. In thecase of TEM₀₁* LDW these fluctuations produce a comparatively smallerchange (by typically a factor of 2) in effective index and hence therequired tolerance on exposure is increased.

The emergence of wavelength division multiplexing (WDM) as the method ofchoice for expanding the bandwidth of existing optical networks meansthat spectrally selective elements are often required in PLC's. In manycases the required PLC functionality is acquired by photo defining Braggreflection phase gratings into the existing PLC channel waveguides. Inthis case the TEM₀₁* and TEM₀₀ channel waveguide exposure conditionswould be determined such that the maximum exposure did not completelysaturate the available photoinduced refractive index change i.e.F_(max)=0.7F_(sat) for example. For efficient Bragg reflection to occurthe photoinduced phase grating should be written uniformly across thewaveguide, requiring a uniform index distribution, n(y), in the channelwaveguide. Clearly channel waveguides produced by TEM₀₁* LDW satisfythis criterion better than channel waveguides formed by TEM₀₀ LDW.

It will be apparent that the application of the method of this inventionto direct writing of channel optical waveguides provides a number ofadvantages over previously known techniques. These include:

a near-uniform lateral refractive index profile.

reduced bend loss in the laser written optical waveguides.

reduction in the effect of exposure fluctuations on waveguideparameters.

reduction of form birefringence in laser written optical waveguides.

the ability to write laterally uniform Bragg graftings by postphotodefinition.

the availability of photoinduced tuning of the waveguide effectiveindex.

the ability to produce optical waveguides with X-junctions requiringdouble the single mode core refractive index in the crossing region.

uniform exposure of photosensitive thin films.

The foregoing describes only some of the variations within the scope ofthis invention and further modification can be made without departingfrom the scope of the invention.

What is claimed is:
 1. A method of directly producing photoinducedchanges in refractive index in selected regions of a photosensitive thinfilm by selectively scanning a laser beam across the surface of thefilm, characterized in that the laser beam impinging the film has anannular substantially circularly symmetric irradiation intensitydistribution.
 2. A method as claimed in claim 1 wherein the radialintensity distribution across the annulus substantially uniform.
 3. Amethod as claimed in claim 1 wherein the annular distribution isprovided by a TEM₀₁* laser beam.
 4. A method as claimed in claim 1including the step of producing the TEM₀₁* laser from a Gaussian laserbeam using a diffracting phase mask technique.
 5. A method as claimed inany one of claims 1 and 4 wherein the film is a planar waveguide andincluding the step of scanning said laser beam so that said photoinducedchanges produce one or more channel waveguides in the photosensitivefilm to create a planar lightwave circuit.
 6. A method as claimed inclaim 5 wherein said waveguides have modes sizes substantially matchedto those of a typical communications optical fibre.
 7. A method asclaimed in claim 5 wherein at least one waveguide includes a curvedpath.
 8. A method as claimed in claim 5 including the step of notcompletely saturating the available photoinduced refractive index changeto provide for subsequent selective photoinduced refractive indexchanges to a portion of or portions of the waveguide.
 9. A method asclaimed in claim 8 including the step of producing said subsequentselective changes to define spectrally selective elements.